1. Field of the Invention
The present invention relates to a process for producing a silicon single crystal. More specifically, the invention relates to a silicon single crystal production process that enables the stable formation of a neck portion.
Priority is claimed on Japanese Patent Application No. 2005-208526, filed Jul. 19, 2005, the content of which is incorporated herein by reference.
2. Description of Related Art
There are a variety of methods for producing silicon single crystals. Of these, the Czochralski (CZ) method is widely used as a silicon single crystal pulling technique capable of commercial mass production.
FIG. 5 is a longitudinal sectional view of an apparatus for pulling silicon single crystal (a silicon single crystal puller) which uses the CZ method. As shown in FIG. 5, growth of the silicon single crystal is carried out in the vessel of a chamber 19, within which a crucible 11 is disposed at a central position. The crucible 11 is composed of a quartz receptacle 11a as an inner layer and, fitted on the outside thereof, a graphite receptacle 11b as an outer layer. A shaft for rotating and vertically raising and lowering the crucible 11 is provided at the base of the outer receptacle 11b of the crucible 11. A cylindrical heater 12 is disposed concentrically about the outer periphery of the crucible 11, and a crystal starting material melted by the heater 12, i.e., a polysilicon melt 13, is held within the crucible 11. Tubular thermal insulation 17 is provided outside of the heater 12 in such a way as to surround the heater 12.
Above the crucible 11, a pull shaft 14 is provided, in a manner that allows the rotation and vertical movement thereof, through a small cylindrically shaped pull chamber 20 formed on and connected to the top of the chamber 19, and a seed crystal 16 is detachably mounted on a seed chuck 15 at the bottom end of the pull shaft 14. The single crystal is grown from a bottom end of the seed crystal 16 by contacting the bottom end of the seed crystal 16 with the surface of the melt 13, then raising the seed crystal 16 while rotating it in the opposite direction to rotation by the crucible 11.
When the seed crystal 16 is brought into contact with the surface of the silicon melt 13 as shown in FIG. 5, the resulting thermal shock generates a high density of dislocations in the seed crystal 16. To pull a silicon single crystal in a dislocation-free state, a neck is provided below the seed crystal 16 and pulling is carried out while using the so-called Dash technique to eliminate generated dislocations from the surface of the crystal and allow dislocation-free crystal to solidify at the bottom end of the neck.
FIG. 6 shows the shape of the neck provided below the seed crystal 16 during pulling by the CZ method. Normally, with regard to the neck 16n that is formed during pulling of the silicon single crystal, as shown in FIG. 6, the seed crystal 16 is brought into contact with the surface of the silicon melt 13 then is pulled upward, causing the bottom end thereof to gradually neck, forming a tapered portion 16n1 of decreasing diameter, following which a constant diameter portion 16n2 of a specific diameter is formed. Hence, the neck 16n is made up the tapered portion 16n1 and the constant diameter portion 16n2. Generally, to make a silicon single crystal dislocation-free, it is thought to be necessary for the constant diameter portion 16n2 of the neck 16n to have a diameter D of about 3 mm or less and a length L of at least 30 mm.
The success rate for eliminating dislocations from a neck 16n having a constant diameter portion 16n2 with a diameter of 3 mm or less is about 70%. Hence, a desire exists for further improvement in this success rate.
Recently, requirements having to do with the efficient production of wafers for semiconductors have increased the need for larger diameter and greater length in the silicon single crystals being pulled. As a result, in the case of 12-inch crystals, for example, it is expected that the weight of the single crystals will exceed 300 kg.
When a silicon single crystal is pulled, the weight of the crystal is supported by the neck. Hence, the entire weight of the single crystal depends from the constant diameter portion of the neck. According to calculations based on the mechanical characteristics of silicon single crystals, supporting a crystal having a large weight in excess of 300 kg would require that the constant diameter portion of the neck have a diameter of at least 4.5 mm. Including also a margin of safety to cover vibrations and shocks in the pulling process, a diameter of 6.0 mm is required. Unless the neck is provided with a strength on this order, there is a strong possibility that the neck will break during pulling of the single crystal, causing the silicon single crystal to drop, which is a serious accident. On the other hand, the success rate for eliminating dislocations from the neck drops sharply when the constant diameter portion has a diameter of 4 mm or more.
JP-A 11-189488 discloses, as a means for preventing the silicon single crystal from falling yet achieving an improved success rate for the formation of a dislocation-free neck, a method in which an auxiliary heating mechanism is used to heat the neck during dislocation elimination by the Dash technique and thereby control the temperature distribution in the neck, alleviating thermal stress that acts on the neck and making it possible to render the silicon single crystal dislocation-free even when the neck has a relatively large diameter.
JP-A 2001-130995 describes a method of stably forming a neck having a diameter of about 4 to 10 mm by controlling the silicon melt temperature and the seed crystal pull rate.
Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 11-189488 (referred to as JP-A 11-189488)
Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. 2001-130995 (referred to as JP-A 2001-130995)
Using the method described in JP-A 11-189488 would require that an auxiliary heating mechanism be included in the apparatus for pulling silicon single crystal, which would have the undesirable effect of increasing the size of the overall crystal pulling system.
As for the method described in JP-A 2001-130995, because the neck has a relatively large diameter of 4 to 10 mm, the success rate for achieving a dislocation-free state is only about 15%. Further improvement in the success rate is thus desired.
It is therefore an object of the invention to provide a silicon single crystal production process which is capable of further improving the success rate for eliminating dislocations.